FIELD OF THE INVENTION AND BACKGROUND
This invention relates to instruments for the non-invasive quantitative and semi-quantitative measurement of constituents in blood, such as blood glucose levels. Specifically, this invention relates to an improved analysis instrument for measurement of the near-infrared energy absorption of a body part, typically a finger, which is being irradiated with near-infrared energy for measurement of the blood analyte levels.
Information concerning the chemical composition of blood is widely used to assess the health characteristics of both people and animals. For example, analysis of the glucose content of blood provides an indication of the current status of metabolism. Blood analysis, by the detection of above or below normal levels of various substances, also provides a direct indication of the presence of certain types of diseases and dysfunctions. In particular, the noninvasive near-infrared quantitative measurement apparatus has particular application for use by diabetics in monitoring the level of glucose in the blood.
U.S. Pat. No. 5,028,787, incorporated by reference herein, teaches the use of near-infrared (NIR) analysis techniques in which NIR radiant energy at bandwidths centering on one or more wavelengths of interest is passed through the skin of a subject. A portion of the energy re-emerges from the subject and is detected by a detector. The detector generates an output signal which is processed in accordance with NIR quantitative analysis algorithms to determine the concentration of selected blood analytes in the subject. The calculated concentration is then displayed on a display device provided on the instrument.
In U.S. Pat. No. 5,077,476, also incorporated by reference herein, it is taught that noninvasive blood glucose measurement can be made by transmitting near-infrared light through an extremity of the body, e.g., the most distal portion of the index finger.
One important aspect of the noninvasive blood analyte measurement technology disclosed in the referenced patents is the ability to calibrate the instruments so that quantitative measurements within acceptable accuracy and precision tolerances may be made over an extended period of time.
One phenomenon that has an affect on the accuracy of such a measurement is the minute expansion and contraction of the finger diameter in correlation with the beating of the heart, known as "pulse beat." This continuous and periodic pulse beat causes an artifact in optical measurement data, as shown in FIGS. 1A and 1B. These figures show a graph of A/D counts received for each "scan" of a subject's finger with a plurality of light wavelengths in sequence, as described generally in the '787 patent.
In such a measurement, light energy is directed into the finger at one side and detected quantitatively at the opposite side, first at a predetermined wavelength y.sub.1, then at a predetermined wavelength y.sub.2, and sequentially to predetermined wavelength y.sub.n. The data for each of the n wavelengths is then used in an algorithm to obtain the quantitative analyte value. In order to reduce the effect of noise on the received data, multiple scans are taken and the data is averaged. Each scan requires many milliseconds. In an example instrument using 14 wavelengths, completion of one scan would require over 100 milliseconds. This period is a significant portion of the period of one pulse beat. A normal human pulse rate is typically between 70 and 100 beats per minute; one pulse beat is thus typically between 600 and approximately 860 milliseconds. Thus, the scan period typically would be between 1/6 and 1/8 of the pulse beat period.
Further, in addition to changing geometry, the finger's chemical constituency changes in a significant manner as blood is periodically forced through it by the heart during each pulse beat; consequently the quantitative light data received by the detector during any one scan have been subjected to varying light absorption conditions. This fact contradicts one of the key assumptions of Beer's Law (which states that the concentration of any organic chemical constituent is proportional to the optical absorption at all wavelengths) that the identical body chemistry is being measured at all wavelengths.
There thus exists a need in the art to eliminate the problems discussed above in achieving acceptable accuracy in noninvasive quantitative blood analyte measurement, which are caused by the spectral interference of pulse beat, and the varying body chemistry present during measurement by different wavelengths within any one scan.